Bilateral Wrist Drop at Presentation in a Child with Spinal Muscular Atrophy Type I

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Spinal muscular atrophy (SMA) is a hereditary degenerative disease affecting the anterior horn cells of the spinal cord and cranial motor nerve nuclei, particularly the hypoglossal nerve, leading to progressive paralysis.1 Based on clinical features, the disease is classified into three types: type I, type II and type III.2 Data suggest that 98% of children with SMA have survival of motor neuron (SMN) gene deletion.3 The diagnosis of SMA is based on clinical features, electromyography, muscle histopathology and genetic studies. With the advent of genetic tests, muscle biopsies are now rarely performed. SMA type I is not an uncommon disorder, with six to eight cases diagnosed every year at the Sultan Qaboos University Hospital in Muscat, Oman.4 Children with SMA type I do not usually survive beyond two years of age.5 Cases of wrist drop (hand drop) are uncommon among paediatric patients. A case of SMA type I in an infant, with bilateral wrist drop at presentation, is described below. A five-week-old male infant presented to Sultan Qaboos University Hospital in April 2013 with decreased movements of the upper and lower limbs. The child had been born with no complications to consanguineous parents. Throughout her pregnancy, the mother had felt normal fetal movements. When the infant was just over one month old, the mother noted that his right hand was weak, with wrist drop. After a week, wrist drop was noted in his left hand and wrist as well. Over the following two weeks the infant’s lower limbs also became weak. The infant’s two older siblings were normal and the mother denied any history of trauma or pressure on the infant’s arms. On examination, the child was alert with normal cranial nerves, other than tongue fasciculations. He had weak respiratory muscles, generalised hypotonia and areflexia. The lower limbs were weaker (grade 1 of 5) than the upper limbs (grades 2–3 of 5), with the proximal limbs weaker than the distal. The patient showed obvious bilateral wrist drop [Figure 1]. Electromyography (EMG) revealed pure axonal motor neuropathy in the upper and lower limbs, with neurogenic EMG confirming the clinical suspicion of SMA. A radial nerve conduction test revealed bilateral motor axonopathy with preserved sensory action potentials and velocity, which is a feature suggestive of a lesion in the anterior horn cells and excludes radial neuropathy. The sensory nerve action potential and the conductions in the other nerves were normal. Although the diagnosis was clear from electrophysiology and the patient’s clinical features, blood test results confirmed that the SMN1 exons 7 and 8 were deleted on both alleles of the gene. Figure 1: Photograph of a five-week-old male infant with spinal muscular atrophy type I, presenting with bilateral wrist drop (left wrist).

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